Herbicide resistance dynamics in a spatially heterogeneous environment

The evolution of herbicide resistance in weeds may depend on three groups of factors: (1) operational factors such as herbicide application in space and time, (2) biological factors related to the ecology and life histories of weeds species, and (3) genetic factors referring to the expression of a benefit/cost balance associated to the mutation conferring resistance. Here we tried to explore this complexity using a simple population genetics based model on a spatial lattice, mixing favourable and unfavourable areas as in a cultivated landscape. The model aims to explore what specificities of the weed biology in interaction with the herbicide application pattern and the genetic characteristics of a resistance trait will favour rapid herbicide resistance spread. The genetic properties especially focused here are the fitness penalty attached to the resistance allele (named the resistance fitness cost) and two genetic dominances, i.e. dominance for fitness cost and dominance for herbicide resistance. Our model suggests that (i) the fate of a resistance allele may depend on the balance between herbicide treatment favouring the resistance and capacities to maintain the resistance gene under untreated years or areas, (ii) changing herbicide application in space will diversely affect resistance dynamics depending on the weed breeding system, and (iii) genetic factors and dominance level for herbicide resistance in particular become predominant for outcrossing weed species only. Overall, the model highlights that the herbicide resistance dynamics is under strong interactions between the different factors, only a part of which being under human control.